Crystallochemistry and electrical properties of Al-doped Sr₂SiO₄ electrolytes

A new Sr₂Si_1−xAl_xO_4−x/2☐_x/2 (x=0, 0.028, 0.06 and 0.075) silicate series has been prepared and the structural and electrical properties were investigated. These samples present three different polymorphs (β, α′_L and α′_H) and the increase of Al-content resulted in a shift of the polymorph content, from a monoclinic form to an orthorhombic one. Aluminium contents higher than x=0.06 did not lead to any further change in the ratio of polymorphs. High temperature X-ray diffraction studies showed a complete stabilisation of α′_L, presenting a superstructure orthorhombic form, at temperatures between RT and 250 °C. At temperatures higher than 500 °C a basic orthorhombic cell is stabilised. Thermogravimetric analysis displayed an increase of the water uptake on doping, due to the generation of oxide vacancies in the lattice. Electrical characterisation in dry and wet atmospheres showed an important proton contribution to the conductivity. In addition, measurements in N₂ and O₂ fluxes displayed a significant p-type electronic contribution to the overall conductivity.